JobID represents the immutable and unique identifier for
the job. JobID consists of two parts. First part
represents the jobtracker identifier, so that jobID to jobtracker map
is defined. For cluster setup this string is the jobtracker
start time, for local setting, it is "local" and a random number.
Second part of the JobID is the job number.
An example JobID is :
job_200707121733_0003 , which represents the third job
running at the jobtracker started at 200707121733.

Applications should never construct or parse JobID strings, but rather
use appropriate constructors or forName(String) method.